1. Field of the Invention
The present invention relates to a printing apparatus a control method for the same.
2. Description of the Related Art
There are known to be printing apparatuses that employ an inkjet printing system. With such printing apparatuses, an image is printed onto a printing medium by discharging ink from an array of orifices on a printhead while moving the printhead back and forth. As a means for discharging ink droplets, such printing apparatuses employ, for example, a method of using air bubbles generated by electrothermal transducers (hereinafter, referred to as “discharge heaters”) such as heater elements. Features of this kind of printing system utilizing heat include, for example, enabling easily reducing the apparatus size and increasing image resolution.
With a printing system that utilizes heat, an electrical signal (hereinafter, referred to as a “pulse”) is applied to the discharge heaters in the printhead so that the electrical signal is converted into thermal energy. This thermal energy is then used to cause film boiling to occur in ink, and ink is discharged using the bubble formation pressure of the air bubbles generated by the film boiling. The discharged ink droplets thus land on a printing medium, and dots are formed on the printing medium.
In such a printing system utilizing heat, it is known that the ink discharge amount fluctuates depending on the viscosity of the ink. Since the ink viscosity changes a large amount depending on the temperature, the ink discharge amount fluctuates depending on the temperature of the ink in the vicinity of the discharge heaters. Specifically, the ink discharge amount increases if the temperature of the ink in the vicinity of a discharge heater is high. This is because the higher the temperature is, the lower the ink viscosity is, and thus the fluidity of the ink improves. Another cause for this is that the growth of the air bubbles formed by film boiling is increasingly promoted as the temperature rises.
For this reason, the temperature of the printhead rises due to the generation of heat by the discharge heaters in printing and the like, and this temperature rise causes the ink discharge amount to increase compared to before the rise in the temperature of the printhead. Thermal energy is also generated due to the application of a pulse to the discharge heaters. For this reason, if the discharge heaters are indiscriminately energized, the temperature is higher the closer to the center in a temperature distribution in the arrangement direction of the discharge heaters, such as in the case of uniformly applying a heating value to a metal rod. As a result, the ink discharge amount is different between high-temperature places and low-temperature places.
In such a case, variation occurs in the diameter of the dots that are formed when ink droplets land on a printing medium, thus leading to the possibility of uneven density in the printed image and degradation in print quality. This problem arises prominently in cases where the discharge heaters are driven with a higher frequency and where the number of orifices is increased in order to meet recent demands for high-speed printing.
Incidentally, with an orifice that has not been used for a certain period of time, the viscosity of ink in the vicinity of the orifice increases (the ink thickens) due to the evaporation of volatile components of the ink from the surface in contact with the air, which may cause the ink to not be discharged satisfactorily. When this phenomenon occurs, an increase in ink concentration and a reduction in ink discharge speed occur particularly at the beginning of printing. In a worst-case scenario, ink may fail to be discharged.
In order to address such a situation, in consideration of the fact that ink viscosity decreases as the temperature rises, it can be said to be effective to reduce the ink viscosity by heating the ink. In light of this, there are known to be mainly two methods of resolving the problem of discharge instability due to ink thickening.
The first is a method of heating the printhead by driving the discharge heaters, and the second is a method of heating the printhead by providing a heater for heating the printhead (hereinafter referred to as a “sub-heater”) separately from the discharge heaters.
With the first method, a pulse according which ink film boiling does not occur, such as a pulse having a short pulse width (hereinafter referred to as a “short pulse”), is applied to the discharge heaters so that the printhead is heated without discharging ink. With the second method, the printhead is heated by applying an arbitrary pulse to the sub-heater.
The technology described below is known as techniques for performing heating using discharge heaters. Japanese Patent Laid-Open No. 10-16228 (hereinafter referred to as “Document 1”) proposes a method of performing heating by applying a short pulse in a non-printing period at an appropriate duty that is in accordance with a printhead temperature condition (the duty being 100% in the case of applying a short pulse with the same frequency as the driving frequency of the printhead during printing). Japanese Patent Laid-Open No. 5-24199 (hereinafter referred to as “Document 2”) proposes a method of performing heating by applying a short pulse to discharge heaters that are not used during printing. Also, Japanese Patent Laid-Open No. 8-336962 (hereinafter referred to as “Document 3”) proposes a method of performing heating in a non-printing period that includes an acceleration region in printhead scanning.
In Document 1, heating is performed by changing the duty as described above, and even when such heating is performed, a pulse is applied to all of the discharge heaters. For this reason, the temperature in the orifice array is not uniform along the orifice arrangement direction, and the temperature is higher in the vicinity of the center of the orifice array than in the vicinity of the ends of the orifice array. Also, with Document 3 as well, the temperature is not uniform in the orifice array similarly to the case of Document 1. Such temperature bias becomes prominent particularly in the case where heating is performed quickly in order to perform high-speed printing.
Furthermore, in Document 2, a short pulse is applied to discharge heaters not used during printing, and actually realizing this processing requires the implementation of circuitry for applying a discharge pulse and a heating pulse at the same time during printing. This results in a cost increase for both the printhead and the apparatus.
With a method of performing heating using discharge heaters in this way, the temperature distribution of the orifice array along the orifice arrangement direction is generally not uniform. For this reason, if an attempt is made to heat all of the discharge orifices to a target temperature or more, the target temperature is overshot in some places. Specifically, when the vicinity of the ends of the orifice array is sufficiently heated, the temperature in the vicinity of the center of the orifice array rises more than anticipated. In other words, it can be said that an excessive amount of power is consumed.
Meanwhile, although there is a method of preventing temperature overshooting by providing a rest period, an excessive amount of time is consumed in this method in order to achieve uniformity in the aforementioned temperature distribution. Also, uniformity cannot be achieved in the temperature distribution, and the ink discharge amount fluctuates, thus causing density unevenness in the printed image particularly at the start of printing.
On the other hand, although the method of performing heating using a sub-heater can achieve greater uniformity in the aforementioned temperature distribution than is possible in the methods of performing heating using discharge heaters, the extra sub-heater, wiring, and the like need to be provided, thus leading to an increase in cost.